ALBERT

Description: Geophysical data from the MEDOC experiment across the Northern Tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic Variscan lithosphere. In contrast, data across the Central Tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup. Here we analyse the MEDOC transect E–F, which extends from Sardinia to the Campania margin at 40.5°N, to define the distribution of geological domains in the transition from the complex Central Tyrrhenian to the extended continental crust of the Northern Tyrrhenian. The crust and uppermost mantle structure along this ~400-km-long transect have been investigated based on wide-angle seismic data, gravity modelling and multichannel seismic reflection imaging. The P -wave tomographic model together with a P -wave-velocity-derived density model and the multichannel seismic images reveal seven different domains along this transect, in contrast to the simpler structure to the south and north. The stretched continental crust under Sardinia margin abuts the magmatic crust of Cornaglia Terrace, where accretion likely occurred during backarc extension. Eastwards, around Secchi seamount, a second segment of thinned continental crust (7–8 km) is observed. Two short segments of magmatically modified continental crust are separated by the ~5-km-wide segment of the Vavilov basin possibly made of exhumed mantle rocks. The eastern segment of the 40.5°N transect E–F is characterized by continental crust extending from mainland Italy towards the Campania margin. Ground truthing and prior geophysical information obtained north and south of transect E–F was integrated in this study to map the spatial distribution of basement domains in the Central Tyrrhenian basin. The northward transition of crustal domains depicts a complex 3-D structure represented by abrupt spatial changes of magmatic and non-magmatic crustal domains. These observations imply rapid variations of magmatic activity difficult to reconcile with current models of extension of continental lithosphere essentially 2-D over long distances.

Description: The magnetic anomaly pattern over the Red Sea can be modeled as a continuous system of sea-floor spreading from the early Miocene to the present by using a time-varying process filter. The half spreading rate is approximately 1 cm/yr (0.4 in./yr) since initial rifting. The parameters that determine the process filter and development of the transition zone are the intrusion parameter (a measure of the dispersion of feeder dikes or horizontal strain about the rift axis), a flow parameter (a measure of the average flow width), and the effusion parameter (a measure of the volcanic effusion and thickness of layer 2). Application of a time-varying process filter appears to be a powerful tool in analyzing magnetic anomalies over passive continental margins. We estimate the flow parameter to be 2.7 km (1.7 mi) and the intrusion parameter to be 7.5 km (4.7 mi) at early rifting. These values suggest that a wide distribution of axial dikes or horizontal strain is the dominant factor in forming the magnetic anomaly pattern. Reduction in the width of the intrusion parameter and the effusion rate as rifting proceeded resulted in focusing of the strain, thinning of layer 2, and formation of the Red Sea deeps. The interpretation of the anomaly pattern implies that basement within the main trough of the Red Sea is composed of a thick accumulation of volcanic sills and flows deposited during early rifting. Extrusion of the lavas occurred over a wide (up to 60 km or 37 mi) zone of strain distributed about the rift axis. The development of this thickened zone of igneous crust is the second of three successive stages of extension for the Red Sea basin. These stages are as follows. (1) An initial stage of crustal stretching or listric faulting in the Oligocene accompanied by probable regional uplift. (2) A period of high volcanic effusion accompanied by the emplacement of stratoid basalts. The stratoid volcanics form overlapping flows and sills with intercalated sediments within the main trough. The spreading center narrowed from the early Miocene to the present. (3) Steady-state sea-floor spreading from Pliocene to present south of a transitional zone near 22°S. The process filter width derived from the magnetic anomaly pattern at earliest rifting can be used to estimate the lateral variation from igneous to continental-type crust in a statistical sense. Our data suggest that the transition from 100% continental crust to 100% igneous or stratoid-type crust should occur within an interval of approximately 32 km (20 mi). Our modeling suggests that phase 2, or the stratoid phase, began about the time of anomaly 5C or chron C5C approximately 16 Ma. This age is compatible with geologic estimates of the initial rifting at the late Oligocene to early Miocene (Coleman, 1974; Gass, 1977). The opening rate for Africa-Arabia plate motion has remained relatively constant since early rifting although the African margin appears to be accreting faster than the Arabian plate.

Description: The Gulf of Cadiz is one of two the test sites chosen for the demonstration of the ESONET - LIDO Demonstration Mission (DM) [1], which will establish a first nucleus of regional network of multidisciplinary sea floor observatories. The Gulf of Cadiz is a highly populated area, characterized by tsunamigenic sources, which caused the devastating earthquake and tsunamis that struck Lisbon in 1755. The seismic activity is concentrated along a belt going from this region to the Azores and the main tsunamigenic tectonic sources are located near the coastline. In the framework of the EU - NEAREST project [2] the GEOSTAR deep ocean bottom multi-parametric observatory was deployed for a one year mission off cape Saint Vincent at about 3200 m depth. GEOSTAR was equipped with a set of oceanographic, seismic and geophysical sensors and with a new tsunami detector prototype. In November 2009 the GEOSTAR abyssal station equipped with the tsunami prototype was redeployed at the same site on behalf of NEAREST and ESONET - LIDO DM. The system is able to communicate from the ocean bottom to the land station via an acoustic and satellite link. The abyssal station is designed both for long term geophysical and oceanographic observation and for tsunami early warning purpose. The tsunami detection is performed by two different algorithms: a new real time dedicated tsunami detection algorithm which analyses the water pressure data, and a seismic algorithm which triggers on strong events. Examples of geophysical and oceanographic data acquired by the abyssal station during the one year mission will be shown. The development of a new acoustic antenna equipped with a stand alone and autonomous acquisition system will allow the recording of marine mammals and the evaluation of environmental noise. References

Description: EGU

Description: Published

Description: Vienna

Description: 1.8. Osservazioni di geofisica ambientale

Description: open